Another Nikon D810A review by Adam Woodworth with few ISO comparisons between the D810A, D810 and D750. Adam's conclusion:
In my experience the D810A matches the high ISO performance of the D750. The D750 has a 24MP sensor and until the D810A came out it was the cleanest high megapixel DSLR, about a stop better than the D810. But the D810A, like the D800/D800E/D810, has a 36MP sensor and appears to have the same high ISO performance of the 24MP D750. With some other nice features, the D810A is an amazing landscape astrophotography camera although there are some gotchas caused but the IR filter.
The D810A is a landmark camera for astrophotography, with amazing high ISO performance and the convenience of (limited) exposures longer than 30s in camera. The IR Cut filter is both a plus and a minus depending on the situation, but the nice pop and increased color that it brings out in the Milky Way is very nice.